The present disclosure relates to the fabrication of micro structures and micro devices.
Micro electrical mechanical devices (MEMS) are often fabricated using planar micro fabrication technologies that usually build micro structures in layers over a substrate in a bottom up manner. The lower layers are deposited and processed, followed by the upper layers. Three dimensional structures can be formed over the substrate by common micro fabrication techniques such as deposition, photo masking, etching, polishing, cleaning, and the use of sacrificial materials.
MEMS often include an electronic circuit for controlling the micro structures. The electronic circuit is usually first fabricated in the substrate. The micro structures are then built over the substrate already containing the electronic circuit. For example, a tiltable micro mirror can be built over a substrate having an electronic circuit for controlling the mirror or a control circuit, in the substrate. The mirror plate of the tiltable mirror includes a reflective upper surface as the mirror surface for reflecting incident light. The control circuit is electrically connected with a conductive surface on the mirror plate and an electrode on the substrate. The tiltable mirror can tilt when the control circuit applies an electric potential between the conductive surface and the surface electrode. The control circuit is often fabricated in one or more layers of complementary metal oxide semiconductor (CMOS) materials.
The above described micro-structure fabrication process can include several challenges. First, the CMOS circuit can suffer heat damages if subsequent processing steps involve temperatures higher than 500° C. In addition, the micro structure often includes a conductive portion to allow the voltage of the conductive portion to be externally controlled, as exemplified by the tiltable micro mirror described above. Furthermore, the applications of the micro structures can impose other constraints in material selection and the fabrication process. For example, some tiltable micro mirrors require high reflectance at the upper surface of the mirror plate but have minimal reflections and scatterings from other surfaces to reduce optical noise and enhance contrast in the image display.